1. Field of the Invention
The field of art to which this invention pertains is the cooling of fluidized particles. It particularly relates to the combustion of combustible material from a particulated solid such as fluidizable catalyst which has been at least partially deactivated by the deposition thereon of a combustible material, such as coke and the cooling of such particles in a vessel that is separate and distinct from the vessel in which such combustion takes place. The present invention will be most useful in a process for regenerating coke-contaminated particles of fluidized cracking catalyst, but it should find use in any process in which combustible material is burned from solid, fluidizable particles.
2. Description of the Prior Art
The fluid catalyst cracking process (hereinafter FCC) has been extensively relied upon for the conversion of starting materials, such as vacuum gas oils, and other relatively heavy oils, into lighter and more valuable products. FCC involves the contact in a reaction zone of the starting material, whether it be vacuum gas oil or another oil, with a finely divided, or particulated, solid, catalytic material which behaves as a fluid when mixed with a gas or vapor. This material possesses the ability to catalyze the cracking reaction, and in so acting it is surface-deposited with coke, a by-product of the cracking reaction. Coke is comprised of hydrogen, carbon and other material such as sulfur, and it interferes with the catalytic activity of FCC catalysts. Facilities for the removal of coke from FCC catalyst, so-called regeneration facilities or regenerators, are ordinarily provided within an FCC unit. Coke-contaminated catalyst enters the regenerator and is contacted with an oxygen containing gas at conditions such that the coke is oxidized and a considerable amount of heat is released. A portion of this heat escapes the regenerator with the flue gas, comprised of excess regeneration gas and the gaseous products of coke oxidation. The balance of the heat leaves the regenerator with the regenerated, or relatively coke free, catalyst.
The fluidized catalyst is continuously circulated from the reaction zone to the regeneration zone and then again to the reaction zone. The fluid catalyst, as well as providing catalytic action, acts as a vehicle for the transfer of heat from zone to zone. Catalyst exiting the reaction zone is spoken of as being "spent", that is partially deactivated by the deposition of coke upon the catalyst. Catalyst from which coke has been substantially removed is spoken of as "regenerated catalyst".
The rate of conversion of the feedstock within the reaction zone is controlled by regulation of the temperature, activity of catalyst and quantity of catalyst (i.e. catalyst to oil ratio) therein. The most common method of regulating the reaction temperature is by regulating the rate of circulation of catalyst from the regeneration zone to the reaction zone which simultaneously increases the catalyst/oil ratio. That is to say, if it is desired to increase the conversion rate, an increase in the rate of flow of circulating fluid catalyst from the regenerator to the reactor is effected. Inasmuch as the temperature within the regeneration zone under normal operations is considerably higher than the temperature within the reaction zone, this increase in influx of catalyst from the hotter regeneration zone to the cooler reaction zone effects an increase in reaction zone temperature. It has become important for FCC units to have the capability to cope with feedstocks such as residual oils and possibly mixtures of heavy oils with coal or shale derived feeds.
The chemical nature and molecular structure of the feed to the FCC unit will affect that level of coke on spent catalyst. Generally speaking, the higher the molecular weight, the higher the Conradson carbon, the higher the heptane insolubles, and the higher the carbon to hydrogen ratio, the higher will be the coke level on the spent catalyst. Also, high levels of combined nitrogen, such as found in shale derived oils, will also increase the coke level on spent catalyst. The processing of heavier and heavier feedstocks, and particularly the processing of deasphalted oils, or direct processing of atmospheric bottoms from a crude unit, commonly referred to as reduced crude, does cause an increase in all or some of these factors and does therefore cause an increase in coke level on spent catalyst.
This increase in coke on spent catalyst results in a larger amount of coke burned in the regenerator per pound of catalyst circulated. Heat is removed from the regenerator in conventional FCC units in the flue gas and principally in the hot regenerated catalyst stream. An increase in the level of coke on spent catalyst will increase the temperature difference between the reactor and the regenerator, and in the regenerated catalyst temperature. A reduction in the amount of catalyst circulated is therefore necessary in order to maintain the same reactor temperature. However, this lower catalyst circulation rate required by the higher temperature difference between the reactor and the regenerator will result in a fall in conversion, making it necessary to operate with a higher reactor temperature in order to maintain conversion at the desired level. This will cause a change in yield structure due to an increase in thermal versus catalytic selectivity which may or may not be desirable, depending on what products are required from the process. Also there are limitations to the temperatures that can be tolerated by FCC catalyst without there being a substantial detrimental effect on catalyst activity. Generally, with commonly available modern FCC catalyst, temperatures of regenerated catalyst are usually maintained below 1400.degree. F., since loss of activity would be very severe at about 1400.degree.-1450.degree. F. If a relatively common reduced crude such as that derived from Light Arabian crude oil were charged to a conventional FCC unit, and operated at a temperature required for high conversion to lighter products, i.e. similar to that for a gas oil charge, the regenerator temperature would operate in the range of 1600.degree.-1800.degree. F. This would be too high a temperature for the catalyst, require very expensive materials of construction, and give an extremely low catalyst circulation rate. It is therefore accepted that when materials are processed that would give excessive regenerator temperatures, a means must be provided for removing heat from the regenerator, which enables a lower regenerator temperature, and a lower temperature difference between the reactor and the regenerator.
The prior art is replete with disclosures of FCC processes which utilize dense or dilute phase regenerated fluid catalyst heat removal zones or heat exchangers that are remote from and external to the regenerator vessel to cool hot regenerated catalyst for return to the regenerator. Examples of such disclosures are as set forth in Daviduk et al. U.S. Pat. No. 4,238,631; Harper U.S. Pat. No. 2,970,117; Owens U.S. Pat. No. 2,873,175; McKinney U.S. Pat. No. 2,862,798; Watson et al. U.S. Pat. No. 2,596,748; Jahnig et al. U.S. Pat. No. 2,515,156; Berger U.S. Pat. No. 2,492,948; Watson U.S. Pat. No. 2,506,123; Lomas U.S. Pat. No. 4,396,531; Lomas et al. U.S. Pat. No. 4,353,812; and Lomas et al. U.S. Pat. No. 4,439,533.
An important consideration in the above FCC processes involving regenerator heat removal is the method of control of the quantity of heat removed. In Harper U.S. Pat. No. 2,970,117 and Huff U.S. Pat. No. 2,463,623, the sole method involves regulation of the rate of flow of regenerated catalyst through external catalyst coolers. This method of heat removal, utilizing external coolers and varying the rate of catalyst circulation through them as the exclusive means of control of the heat exchanger duty, involves the continual substantial changing of the catalyst loading on the regenerator with the associated difficulty or impossibility of maintaining convenient steady state operations. In an improved method of using a remote cooler, disclosed in Lomas et al. U.S. Pat. No. 4,353,812, the heat transfer coefficient across the heat transfer surface is controlled by varying the catalyst density through regulation of fluidizing gas addition. The '812 reference also shows the use of a vent line at the top of the catalyst cooler in addition to a catalyst withdrawal line. U.S. Pat. No. 4,615,992, issued to Murphy, also shows the use of a vent line to transfer relatively catalyst-free gas from the top of a remote catalyst cooler to a regenerator vessel. In both cases the cooler receives a high catalyst flux (catalyst flux is the weight of catalyst flowing through a given cross-section per unit of time) through the standpipe feeding the cooler which prevents a catalyst and air mixture from flowing countercurrently up the standpipe. One method of control that has been purposefully avoided in the operation of most heat removal zones is the circulation rate of cooling medium. In order to prevent overheating and possible failure of the cooling tubes, cooling medium usually circulates through the tubes at a high and constant rate. Therefore, the most common form of catalyst coolers uses a net flow of catalyst through the cooler and for this reason is termed a flow through cooler. Heat transfer in these flow through coolers is controlled by regulating the net flow or inventory of catalyst either alone or in combination with regulation of the fluidization gas addition.
The principle of controlling heat removal with fluidizing gas addition is used in Lomas U.S. Pat. No. 4,439,533 to operate what is herein referred to as a backmixed cooling zone. In a backmixed cooling zone, catalyst to be cooled circulates in and out of a cooler inlet opening without a net transport of catalyst through the cooler. The difference between a flow through cooler operation and a backmix cooler operation is that in the backmix operation all of the catalyst circulation into and out of the cooler is through the same opening whereas in a flow through operation catalyst is transported in at least one direction down the length of the cooler. U.S. Pat. No. 2,492,948, issued to C. V. Berger, depicts a catalyst cooler that communicates with the lower portion of an FCC regenerator and superficially resembles a backmix type cooler; however, Berger is really a flow through type cooler since it receives catalyst through an annular opening, transports catalyst down an internal annular passage, transports catalyst up through a heat transfer passage, and ejects catalyst from a central opening. The addition rate of fluidizing gas to the catalyst is the sole variable for controlling the amount of heat transfer in the backmix type cooler. The fluidizing gas addition rate controls the heat transfer coefficient between the catalyst and the cooling surface and the turbulence within the cooler. More turbulence in the backmix cooler promotes more heat transfer by increasing the interchange of catalyst at the cooler opening and increasing the average catalyst temperature down the length of the cooler. A remote backmix cooler has the advantage of a simple design and is readily adapted to most FCC configurations since it requires a single opening between the regenerator and the cooler. Unfortunately, backmix coolers often have the drawback of lower heat transfer duty in comparison to flow through type coolers, especially in the case of backmix coolers that are horizontally displaced from a regeneration vessel.
Several more recent patents have taught methods of improving the control of backmix coolers. U.S. Pat. No. 4,923,834 issued to Lomas teaches the use of a aeration controlled transfer line to improve the circulation of catalyst around the top of a backmix cooler. The use of a lower backmix cooler in combination with an upper flow through catalyst cooler is shown in U.S. Pat. No. 4,881,592 issued to Cetinkaya.
However all such backmix coolers been arranged with the source of hot catalyst at the top of the cooler and the heat exchange surface of the cooler located below the source of circulating catalyst. This arrangement has several disadvantages. For example a downwardly extending backmix zone collects debris in the bottom of the cooler. Such debris blocks the catalyst flow through the typical tube bundle of the cooler and reduces its available heat exchange duty. In addition aeration gas injected between the heat exchange tubes in a downwardly extended backmix cooler sometimes erodes the tubes by blasting catalyst against the tube wall surfaces. Moreover, in the case of retrofitting catalyst coolers into existing FCC units, a large amount of equipment fills the limited space around the bottom of an FCC unit and leaves little space to extend the lower part of a cooler into such an area.